A MEMS (micro electromechanical system) device is used, for example, in the manufacture of display devices that are controlled by digital information. There are currently in development, and also in use, a number of alternatives to the standard CRT (cathode ray tube) display that has for years been a common manner of displaying visual images and text in such applications as televisions and computer monitors. These new types of displays vary somewhat with respect to each other, but are often superior to CRT displays in that they are more compact, or produce a superior visual image, or both. Digital Light Processing®, or DLP®, commercially available from Texas Instruments of Dallas, Tex., is one such alternative. DLP® is used, for example, to produce the visual displays in high-definition television (HDTV) applications.
Naturally, these different types of displays differ primarily with respect to the method by which the image is produced. In a DLP® system, a MEMS device called a DMD (digital micro-mirror device) typically performs a key part of this function. The DMD modulates, that is, alters the characteristics of, light received from a light source by selectively reflecting portions of the received light beam to create an image. FIG. 1 is a partial aerial (plan) view of a DMD 100, exemplary of one that may be used in a DLP® system. The portion of DMD 100 illustrated in FIG. 1 shows micro-mirrors 24 through 29 (partially-shown mirrors are not numbered). Mounting vias 30 through 35 are formed to mount the micro-mirrors 24 through 29, respectively. Note that in this illustration it is assumed that the micro-mirrors are substantially the same and mounted in the same way, so only one of them will be discussed in detail.
Micro-mirror 25 for example, is a very small mirror having a reflecting surface 125. It is mounted by via 31 onto a hinge 131 that allows micro-mirror 25 to tilt in two or more directions. The direction at which each individual micro-mirror is tilted at any given moment determines the direction in which light striking the mirror surface 125 will be reflected. The direction of mirror tilt is controlled by small electrical voltages alternately applied to posts 135 and 140, which either attract or repel a nearby portion of the micro-mirror 25, causing it to retain or to change its orientation. A memory cell (not shown) located underneath micro-mirror 25 allows the controlled-voltage operation, and the memory cell is in turn driven by a controller located elsewhere (and also not shown in FIG. 1). Note that although only six micro-mirrors are fully shown in FIG. 1, the typical DMD device contains thousands—often on the order of one million of such devices fabricated onto a square or rectangular-shaped die (see FIG. 3) that itself typically measures only between one and two centimeters on a side.
The cumulative effect of reflecting light off of these selectively controlled micro-mirrors is to create an image. This image, naturally quite small, is then passed through a projection lens to convert it to an appropriate size for viewing. FIG. 2 is simplified block diagram illustrating an exemplary optical display system 200 that may be assembled using DMD 100 of FIG. 1. Light from a light source 211, which may be an arc lamp or an LED, is collimated and directed along a first portion 221 of the optical path 210. A color wheel 213 is used to produce selectively-colored light for producing colored images. The condenser lenses 212 and 214 shape the beam of light as it propagates along the first portion 221 of optical path 210. The selectively-colored light eventually falls on the DMD 100, where it is transformed into a visual image. The visual image created by DMD 100 is directed to a second portion 222 of the optical path 210, which includes a display screen 219, which may, for example, be an HDTV screen, presents the visual image display intended to be seen by the viewer. The projection lens 218 enlarges the image created by DMD 100 so it will fit the display screen 219.
In order to manufacture DMD 100 and similar MEMS devices, a modified form of the standard semiconductor fabrication process may be used. FIG. 3 is an aerial view of a semiconductor wafer 300 used in such a process. This wafer is typically a very thin slice from an ingot of silicon or some other suitable material. The surface 301 of semiconductor wafer 300 is then populated, through a number of fabrication process steps, with many electrical, and the case of a MEMS device, electromechanical components. These components are formed by a series of steps that use such methods as ion implantation, the deposition of layers of new materials, and patterned etching of the various created surfaces. Many of these processes are automated or semi-automated for both efficiency and precision.
The tiny components formed on the semiconductor wafer, such as micro-mirrors, memory cells, and transistors, combine to form a chip, which is an independently functioning device for use in applications such as DLP®. Because these chips are so small, a large number of them may be formed on a single wafer such as wafer 300. In FIG. 3, wafer 300 is shown to be populated with thirty-six chips, although in practice there tends to be a greater number of them. Each chip is situated on a portion of wafer 300 that is sometimes called a die. When all or most of the fabrication steps have been completed, the dice are separated in a process known as singulation. Each of the separated devices, or at least those that have passed inspection, may then be used in an application such as the projection display system 200 of FIG. 2.
Needless to say, singulation is an important part of the fabrication process. The dice must be separated from each other in such a manner so as not to damage chip components. Singulation is frequently performed in a multi-step sawing and breaking process (described in greater detail below) that is designed to ensure device integrity to the greatest extent possible.
MEMS device chips such as those used for DMDs pose a somewhat unique challenge in the singulation portion of the fabrication process. When they are used for optical applications such as projection display system, for example, one surface of the chip must be able to receive and to reflect light for the chip to perform its function. Because the reflecting surface is actually made up of thousands of tiny micro-mirrors that are continually being reoriented, the reflecting surface is provided with physical protection in the form of a cover made of a glass or some similar material. The glass cover typically includes a cover wafer, that is, a relatively flat plate that is separated slightly from the chip reflecting surface in order to allow the micro-mirrors to operate. This separation is often achieved using an interposer layer (or wafer). The interposer layer typically forms a grid so that the reflecting surface for each chip is left exposed and the light path to it is unobstructed. Along with the wafer itself, the cover wafer and the adjacent members of the interposer grid form a sealed recess above the reflecting surface.
This wafer assembly configuration is illustrated in FIG. 4. FIG. 4 is an elevation (side) cross-sectional view of an exemplary MEMS wafer assembly 400. The view is taken from the perspective of section line 4-4 shown in FIG. 3 (note that FIG. 3 itself shows only the wafer 300). In the view of FIG. 4, six substantially identical active areas are visible, and exemplary one of which is numbered 435. Each active area includes the many micro-mirrors such as those illustrated in FIG. 1. At the top of exemplary active area 435 is formed a reflecting surface 430 (also enumerated in FIG. 3). As can be seen in FIG. 4, wafer 300 is overlaid with an interposer grid layer 410, which is secured in place using an adhesive material (not shown). Note that only the latitudinal grid members, for example members 412 and 414, are shown in FIG. 4; the longitudinal grid members are for clarity omitted. Note also that the terms ‘latitudinal’ and ‘longitudinal’ (and correspondingly the ‘rows’ and ‘columns’ of dice shown in FIG. 3) are used herein for convenience but are arbitrarily chosen and distinguishable only relative to one another unless otherwise noted or apparent from the context.
A glass cover wafer 420 is mounted to interposer layer 410, typically likewise using an adhesive (not shown). Together glass cover wafer 420 and interposer layer 410 may be said to form a cover or cover assembly 415. The cover assembly may be formed of two or more components, as shown in FIG. 4, or may be formed as a unit. In any case, care must be taken when singulation is performed so as not to damage the cover assembly 415 so that the recess 425, the space created above active area 435 (as well as the other, similar recesses), remains sealed against intrusion by water vapor or other deleterious materials. Of course, undue damage to the wafer 300 must be avoided as well.
Current wafer singulation methods approach this challenge by using a partial-saw and break method. A saw cut creates a fault-line so that the dice may be separated using an impact tool. For MEMS dice this can be a somewhat complicated and cumbersome procedure. If a method could be devised to singulate MEMS dice using a relatively-simple procedure that nevertheless achieves a higher product yield and reliability, production costs could be reduced. The method of the present invention provides just such a solution.